No Compound Compound A Compound B Compound C 10 mg/mL 50 mg/mL 10 mg/mL 50 mg/mL
ID: 209538 • Letter: N
Question
No Compound
Compound A
Compound B
Compound C
10 mg/mL
50 mg/mL
10 mg/mL
50 mg/mL
10 mg/mL
50 mg/mL
# of his+colonies
5
5
4
48
88
101
189
12. (1pt) Which of the following is the likely cause of histidine revertants observed in the Ames test in the above question? Choose ALL that apply
insertion or deletions due to replication slippage
single-nucleotide base changes
synonymous mutations
double-strand DNA breaks
pyrimidine dimers
No Compound
Compound A
Compound B
Compound C
10 mg/mL
50 mg/mL
10 mg/mL
50 mg/mL
10 mg/mL
50 mg/mL
# of his+colonies
5
5
4
48
88
101
189
Explanation / Answer
Before answering this question, let us understand the principle of Ames test.
Ames test is used for the screening of chemical mutagens which can adversely affect the DNA by causing mutations. The most common bacterium used in Ames test is, Salmonella typhimurium with mutations in genes involved in histidine synthesis (also called auxotrophic mutant). This organism will be grown in presence of a potential carcinogen and histidine. If the carcinogen causes mutation, it reverts the bacteria to grow in a histidine medium suggesting the possibility of mutation in the organisms DNA, induced by the carcinogen.
It is interesting to note that Ames test specifically detects the following mutations:
a. Frame-shift mutations
b. Point mutations
c. Missense mutations
d. Lesions on DNA
Now let us answer the given question using the above information.
The likely causes of histidine revertants observed in the Ames test are as follows:
Single nucleotide base changes - As explained above, it is also called point mutation. So, this could be one of the reasons for the occurrence of histidine revertants.
Double strand breaks - This is nothing but a DNA lesion and can also be induced by the carcinogen. Hence, this could be one of the reasons for the occurrence of histidine revertants.
Pyrimidine Dimers - This is also another kind of DNA lesion and can be caused by the carcinogen. So, this is also the reason for the occurrence of histidine revertants.
Why are other options not correct for this question?
Insertion or deletion due to replication slippage is generally not caused by carcinogens.
Synonymous mutations lead to the expression of same amino acid. However, only the triplet codon changes. Example: GGT codes for glycine. If GGT changes to GGG, this will also code for glycine. So, synonymous mutations cannot lead to the emergence of histidine revertants. Hence this option is not correct.
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